I've come close to finishing my first 10.5 inch mirror. I'm still really uncertain about readings from my Foucault tester, and I also use a Ronchi Grating to test the surface. One appears to read overcorrected, and the other reads undercorrected. The next step is star testing but cloudy nights have delayed that. My question is that with the mirror in the scope I can distinguish individual pine needles from trees 200 yds. away. Does this mean that the mirror is good or does this really indicate very little about the mirror? Thanks, Chuck
North Carolina is too far away to invite you to bring it over (To California) for a test.
It is hard to say what is happening on the mirror. It may be a difference in temperature from one reading to the next. It may be a slight difference in your interpertation (or non-repeatability of your technique). Are you dealing with an astigmatism, or are you referring to a whole mirror test here? What software do you use to reduce your Foucault data to give you your results. The program I am most familiar with lets you put in three sets of readings and average them out. Unless you are pretty good, and your tester is pretty good, it is easy to get slight variations between tests.
I do know that a mirror is never done. You just eventually have to pry it from the cold, dead hands of the optician and put it in a telescope eventually. The ATM'er will want to continue to tweak that last little bit of perfection, while the observer just wants to look through the thing.
You don't mention the focal length of your mirror. The Ronchi test is less precise with faster mirror - less than about F6.
The Foucault will show surface roughness, turned-edge, and zonal errors by direct inspection. To test for spherical aberration, test results depend a lot on how skilled you are at matching the brightness of opposing zones on the mirror face while moving the knife-edge back-and-forth. I tested a bunch of completed mirrors (commercial and homemade) to increase my skill-level before trying to use it to figure my own mirrors.